1. Field of the Invention
The present invention relates to a zoom lens system, which is suitable for an image taking optical system of a digital still camera for example.
2. Related Background Art
In recent years, along with promotion of high performance image pickup apparatus (camera) such as a video camera or a digital still camera using a solid-state image pickup element such as a CCD sensor or a CMOS sensor, a zoom lens having a large aperture ratio including a wide angle of view is required for an image taking optical system used in the image pickup apparatus.
In a camera of this sort, various kinds of optical components such as an optical low-pass filter and a color correction filter are disposed between a lens rear end portion and an image pickup element. Hence, a lens system having a relatively long back focus is required for an image taking optical system used in that camera. Moreover, in a case of a color camera using an image pickup element for a color image, in order to avoid color shading, an optical system which has excellent telecentric characteristics on an image side is desired as an optical system used in the color camera.
Heretofore, there have been proposed various two-unit zoom lenses of a so-called short zoom type. Such a two-unit zoom lens includes a first lens unit having a negative refractive power and a second lens unit having a positive refractive power and carries out zooming while a lens distance between the first and second lens units is changed. In those zoom lenses of the short zoom type, a changing magnification is carried out by moving the second lend unit having the positive refractive power, and compensation for an image position accompanying the changing magnification is carried out by moving the first lens unit having the negative refractive power. In a lens construction including those two lens units, a zoom ratio is about 2.
In order to make the overall lens compact while a high zoom ratio equal to or larger than 2 is maintained, a so-called three-unit zoom lens in which a third lens unit having a negative or positive refractive power is disposed on an image side of the second-unit zoom lens to carry out various aberrations generated due to an increase in the zoom ratio is proposed in JP 7-3507 B or JP 6-40170 B for example.
The three-unit zoom lens fulfilling the back focus and the telecentric characteristics is proposed in Japanese Patent Application Laid-Open No. S63-135913 or Japanese Patent Application Laid-Open No. H07-261083 for example. In addition, Japanese Patent Application Laid-Open No. H03-288113 discloses an optical system in which the first lens unit having the negative refractive power is fixed in the three-unit zoom lens, and the zooming operation is carried out by moving the second lens unit having the positive refractive power and the third lens unit having the positive refractive power. Likewise, the three-unit zoom lenses are also disclosed in Japanese Patent Application Laid-Open No. H07-261083, U.S. Pat. No. 4,999,007, Japanese Patent Application Laid-Open No. H11-23967, Japanese Patent Application Laid-Open No. H11-287953, and Japanese Patent Application Laid-Open No. 2001-296475.
The three-unit zoom lens disclosed in JP 7-3507 B or JP 6-40170 B is mainly designed for 35-mm film photography. Hence, it is hardly to say that the length of the back focus and the excellent telecentric characteristics which are required for the optical system using the solid-state image pickup element are compatible with each other.
In the case of the zoom lens disclosed in Japanese Patent Application Laid-Open No. S63-135913, Japanese Patent Application Laid-Open No. H07-261083, or Japanese Patent Application Laid-Open No. H03-288113, since the number of elements of lenses constituting each of the lens units is relatively large, a total lens length is long, and manufacturing cost is high.
Moreover, in the zoom lens disclosed in Japanese Patent Application Laid-Open No. H07-261083, a positive lens is disposed in a position nearest an object side of the first lens unit having the negative refractive power. Thus, in particular, when an angle of view is increased, the zoom lens tends to increase a lens outer diameter. Furthermore, this example often complicates a mechanical construction combined with the movement during the zooming operation since the focusing for a neat object is carried out by moving the first lens unit having the negative refractive power.
Though the zoom lens disclosed in U.S. Pat. No. 4,999,007 has a simple construction in which each of the first and second lens units is constituted by one element of single lens, a total lens length in a wide angle end is relatively long. Moreover, since the first lens unit and an aperture stop are largely distant from each other in the wide angle end, an incident height of an off axis principal ray is large and hence a diameter of each of the lenses constituting the first lens unit is increased. For this reason, the zoom lens easily scales up a construction of the overall lens system.
The zoom lens disclosed in Japanese Patent Application Laid-Open No. H11-23967 A has the zoom ratio equal to or larger than 2.5. However, since an exit pupil distance enough to be suitable for the solid-state image pickup element is secured after sufficient optical performance is ensured from a wide angle end to a telephoto end, it is hardly to mention that the sufficient compactness is made in terms of a total lens length.
In the zoom lens disclosed in Japanese Patent Application Laid-Open No. H11-287953, each of the first and second lens units is constituted by three elements of single lens, thereby realizing compactness. Also, in the zoom lens disclosed in Japanese Patent Application Laid-Open No. 2001-296475, the first lens unit is constituted by two elements of lenses, thereby realizing further compactness. However, either of those zoom lenses has the zoom ratio of about 3. The above documents hence do not disclose preferable constructions when the higher zoom ratio is to be realized.